The present invention relates to a motor drive control system suitable for use for controlling the drive of a motor, such as a brushless motor or a linear motor, each of which has a plurality of excitation phases, through the use of rectangular waves.
For example, a brushless motor, which has been employed as a drive source for a power steering apparatus of a motor vehicle, is a motor having three or more excitation phases, with driving being performed by rectangular excitation currents.
In the case of a five-phase brushless motor, in general, a motor drive circuit is made to rotationally drive a rotor of this motor in a manner to cause excitation with rectangular currents while successively switching five-phase (a-phase, b-phase, c-phase, d-phase and e-phase) exciting coils a to e, disposed to surround an outer circumferential surface of the rotor in a state separated by an electrical angle of 72xc2x0, by phases according to a four-phase exciting method of performing the four-phase excitation simultaneously under control by a control circuit comprising a microcomputer or the like. In this four-phase exciting method, motor current always flows to four of five phases, while, for supplying the current to each of the phases in a well-balanced condition, all the resistances of the exciting coils are designed to be equal to each other. Incidentally, in the four-phase exciting method for the five-phase brushless motor, of the five phases, a phase to which the motor current flows is referred to as an xe2x80x9cON-phasexe2x80x9d, and a phase to which it does not flow is called an xe2x80x9cOFF-phasexe2x80x9d.
Such a motor drive circuit is made up of 10 field effect transistors (FETs). These 10 transistors constitute 5 series transistor circuits, each of which is constructed by connecting two transistors, corresponding to each other, in series, and are connected between the positive and negative terminals of a power source. Further, the connecting parts of the two transistors of each of the series transistor circuits are coupled to the external terminals of 5 exciting coils xe2x80x9caxe2x80x9d to xe2x80x9cexe2x80x9d, connected in a Y-configuration, thus establishing a connection with a coil circuit of the motor.
For instance, the direction and length of an exciting current to be supplied from this motor drive circuit to each of the exciting coils is as shown in FIG. 1 with respect to the value of a rotating angle (electrical angle) of the rotor. That is, the exciting coils are successively switched by phases at an interval of 36xc2x0 in electrical angle, and each phase coil is excited for 14xc2x0 in electrical angle so that the rotor rotates continuously. In FIG. 1, when the electrical angle is taken as xcex8, the intervals of 0xe2x89xa6xcex8 less than 36xc2x0, 36xc2x0xe2x89xa6xcex8 less than 72xc2x0, 72xc2x0xe2x89xa6xcex8 less than 108xc2x0, 108xc2x0xe2x89xa6xcex8 less than 144xc2x0, 144xc2x0xe2x89xa6xcex8 less than 180xc2x0, 180xc2x0xe2x89xa6xcex8 less than 216xc2x0, 216xc2x0xe2x89xa6xcex8 less than 252xc2x0, 252xc2x0xe2x89xa6xcex8 less than 288xc2x0, 288xc2x0xe2x89xa6xcex8 less than 324xc2x0 and 324xc2x0xe2x89xa6xcex8 less than 360xc2x0 are expressed by (1), (2), . . . , (10).
In this instance, the a-phase current flows in the positive direction in the intervals (1) and (2), becomes xe2x80x9c0xe2x80x9d in the interval (3), flows in the negative direction in the intervals (4) to (7), becomes xe2x80x9c0xe2x80x9d in the interval (8), and again flows in the positive direction in the intervals (9), (10) and (1). The b-phase current flows in the positive direction in the intervals (1) to (4), becomes xe2x80x9c0xe2x80x9d in the interval (5), flows in the negative direction in the intervals (6) to (9), becomes xe2x80x9c0xe2x80x9d, and again flows in the positive direction in the interval (1). The c-phase current flows in the negative direction in the interval (1), becomes xe2x80x9c0xe2x80x9d in the interval (2), flows in the positive direction in the intervals (3) to (6), becomes xe2x80x9c0xe2x80x9d in the interval (7), and again flows in the negative direction in the intervals (8) to (10) and (1). The d-phase current flows in the negative direction in the intervals (1) to (3), becomes xe2x80x9c0xe2x80x9d in the interval (4), flows in the positive direction in the intervals (5) to (8), becomes xe2x80x9c0xe2x80x9d in the interval (9), and again flows in the negative direction from the interval (10). The e-phase current assumes xe2x80x9c0xe2x80x9d in the interval (1), flows in the positive direction in the intervals (2) to (5), becomes xe2x80x9c0xe2x80x9d in the interval (6), flows in the positive direction in the intervals (7) to (10), and again becomes xe2x80x9c0xe2x80x9d in the interval (1). Accordingly, at the boundary (the switching point at every 36xc2x0 in electrical angle) of each of the intervals (1) to (10), two of five exciting coils are switched in opposite directions.
Although this switching of the excitation current can be expressed in principle by the leading edges and trailing edges of the rectangular waves as shown in FIG. 1, in fact the leading edges and the trailing edges do not vary at right angles with respect to its horizontal axis, and some amount of time xcex94t (approximately three times the time constant of the motor circuit) is needed until the exciting current rises in the positive direction or falls in the negative direction. For example, at the boundary (288xc2x0 in electrical angle) between the interval (8) and the interval (9) in FIG. 1, the a-phase current rises from xe2x80x9c0xe2x80x9d to a positive constant value, while the d-phase current falls from a positive constant value to xe2x80x9c0xe2x80x9d, and both the b-phase and c-phase currents assume a negative constant value, and even the e-phase current is at positive constant value. The variations at these boundary portions are shown enlarged in FIG. 2.
In detail, the a-phase rising (first transition) current increases gradually from xe2x80x9c0xe2x80x9d to the positive constant value for the time xcex94t, while the d-phase falling (last transition) current decreases from the positive constant value to xe2x80x9c0xe2x80x9d for time xcex94t1 shorter than the time xcex94t (less than the time constant of the motor circuit). At this time, the other three phases, the b-phase, c-phase and e-phase are not intended to vary. When the five-phase currents are expressed with ia, ib, ic, id and ie, the following relationship occurs among these currents.
ia+id+ie=xe2x88x92(ib+ic)=Ixe2x80x83xe2x80x83(1)
Thus, when the a-phase and d-phase currents vary as mentioned above, the b-phase, c-phase and e-phase currents also vary. That is, since the rates of current change in the a-phase and the d-phase differ from each other, the sum of the two phase currents does not assume a constant value, and as a result of the variation of the b-phase and c-phase currents shown in FIG. 2, the e-phase current also varies for the aforesaid time xcex94t. This current variation causes a transient torque variation.
The above-mentioned difference between the rates of current change of the rise and fall of the two phase currents is based upon the following principle. First, let it be assumed that a power supply voltage to be given to the motor drive circuit is taken to be Vb and the voltage at the central connection point of the exciting coils xe2x80x9caxe2x80x9d to xe2x80x9cexe2x80x9d which are connected in a radiating arrangement is taken as Vn. Further, the interval of the time xcex94t1 is indicated by {circle around (1)} and the interval of the time xcex94t2 (=xcex94txe2x88x92xcex94t1) is indicated by {circle around (2)}.
In the interval indicated by {circle around (1)}, the d-phase (OFF-phase) current id, switching from the positive to xe2x80x9c0xe2x80x9d, falls from half (I/2) of energizing current I, to be supplied from the motor drive circuit to the motor, to xe2x80x9c0xe2x80x9d at a rate of change depending upon xe2x88x92Vn, a reverse electromotive voltage Ed of the coil and a time constant of the motor circuit. At this time, if a voltage to be applied to an OFF-phase equivalent circuit is taken to be VOFF, VOFF=xe2x88x92Vnxe2x88x92Ed less than 0, and Vn becomes nearly Vb/2. On the other hand, the a-phase (ON-phase) current ia rises from xe2x80x9c0xe2x80x9d at a rate of change depending upon a voltage Vb, xe2x88x92Vn, a reverse electromotive voltage Ea of the coil and a time constant of the motor circuit. At this time, if a voltage to be applied to an ON-phase equivalent circuit is taken to be VON, VON=Vbxc2x7Duty1 (a duty ratio of a rectangular wave)xe2x88x92Vnxe2x88x92Ea. Explaining with equations, through the OFF-phase equivalent circuit, the current id is expressed by the following equation (2), where T denotes an electrical time constant of the equivalent circuit and R depicts a resistance of the equivalent circuit.
id(t)=I/2xc2x7exe2x88x92t/T+VOFF/Rxc2x7(1xe2x88x92exe2x88x92t/T)xe2x80x83xe2x80x83(2)
Accordingly, when t=0, id=I/2.
On the other hand, through the ON-phase equivalent circuit, the current ia is expressed by the following equation.
ia(t)=VON/Rxc2x7(1xe2x88x92exe2x88x92t/T)xe2x80x83xe2x80x83(3)
Accordingly, when t=0, ia=0, and when txe2x86x92∞, ia=VON/R=I/2. Thus, the rates of change of the currents id and ia in the OFF-phase and the ON-phase are given by the following equations (4) and (5), respectively.                                                                                           ⅆ                                                            i                      d                                        ⁡                                          (                      t                      )                                                                      /                                  ⅆ                  t                                            =                              xe2x80x83                            ⁢                                                                    -                                          (                                              1                        /                        T                                            )                                                        ⁢                                      (                                          I                      /                      2                                        )                                    ⁢                                      ⅇ                                                                  -                        t                                            /                      T                                                                      +                                                      (                                          1                      /                      T                                        )                                    ⁢                                      (                                                                  V                        OFF                                            /                      R                                        )                                    ⁢                                      ⅇ                                                                  -                        t                                            /                      T                                                                                                                                              =                              xe2x80x83                            ⁢                                                -                                      (                                                                  I                        /                        2                                            -                                                                        V                          OFF                                                /                        R                                                              )                                                  ⁢                                  (                                      I                    /                    T                                    )                                ⁢                                  ⅇ                                                            -                      t                                        /                    T                                                                                                                          =                              xe2x80x83                            ⁢                                                -                                      (                                                                  I                        /                        2                                            +                                              Vn                        /                        R                                            +                                                                        E                          d                                                /                        R                                                              )                                                  ⁢                                  (                                      1                    /                    T                                    )                                ⁢                                  ⅇ                                                            -                      t                                        /                    T                                                                                                          (        4        )                                                                                                      ⅆ                                                            i                      a                                        ⁡                                          (                      t                      )                                                                      /                                  ⅆ                  t                                            =                                                (                                      1                    /                    T                                    )                                ⁢                                  (                                                            V                      ON                                        /                    R                                    )                                ⁢                                  ⅇ                                                            -                      t                                        /                    T                                                                                                                          =                                                (                                      I                    /                    2                                    )                                ⁢                                  (                                      1                    /                    T                                    )                                ⁢                                  ⅇ                                                            -                      t                                        /                    T                                                                                                          (        5        )            
In the above equations (4) and (5), since (I/2+Vn/R+Ed/R) greater than I/2, the rate of current change in the OFF-phase is greater than the rate of current change in the ON-phase. In particular, when the resistance R of the equivalent circuit is low, when the power supply voltage Vb (≈2Vn), or when the counter electromotive voltage Ed at a high operation speed is high, the rate of current change in the OFF-phase is considerably greater than the rate of current change in the ON-phase. Accordingly, the time (xcex94t) to be taken until the ON-phase current ia rises from xe2x80x9c0xe2x80x9d to I/2 is longer than the time (xcex94t1) to be taken until the OFF-phase current id falls from I/2 to xe2x80x9c0xe2x80x9d. That is, at the end of the interval indicated by {circle around (1)}, the ON-phase current ia does not reach I/2, but is still in the process of rising.
Following this, in the interval indicated by {circle around (2)}, the ON-phase current ia finally reaches the constant value I/2, with a time xcex94t2 (twice to three times the time constant of the motor circuit) being necessary to reach it. For this reason, the rates of current change differ at the rise and fall of two phase currents when they are switched.
As described above, in the exciting current control by the conventional motor drive circuit, since the rates of change at the rise and fall of the currents for two phases (for example, the a-phase and the d-phase in FIG. 1) to be switched differ from each other, the non-switched phase currents (for example, the b-, c- and e-phases) vary, which causes transient torque variation.
For suppressing such current variation generating the torque variation at the phase switching, the control of the current for each phase will do. However, in this case there is a need to detect the current for each phase for that control, and 2 or more current detecting circuits become necessary. In particular, since the five-phase brushless motor has employed the four-phase exciting method, there has been a problem in that the motor drive circuit requires four current detecting circuits and four current loops, which leads to a complicated configuration of the drive circuit and to further increases in cost.
Meanwhile, FIG. 3 is a characteristic diagram showing a phase current waveform for each exciting coil and a torque waveform in a conventional five-phase brushless DC motor drive control system. As is obvious from this illustration, the end of the OFF-phase energizing period takes place by the time the next commutation begins.
However, in the case of the conventional brushless DC motor drive control system, although the end of the period of the energization by the pulse width modulation (PWM) drive takes place by the time the next commutation begins, when the rotational speed of the motor is low (if the time between two commutations is long), the OFF-phase current already comes into an intermittent current mode in the PWM drive before the next commutation begins, while the OFF-phase residual current approaches xe2x80x9c0xe2x80x9d but does not reach xe2x80x9c0xe2x80x9d. The residual current still flows continuously except that the OFF-phase energization completely comes to an end, and the electromagnetic torque due to that residual current shows an effect of reducing the electromagnetic torque of the entire motor.
Thus, at the time the next commutation begins, rapid level variation occurs at originally continuous portions in the torque waveform as shown in FIG. 3. Especially in cases where the motor torque constant is high and the residual current is large, the rapid level variation of the torque becomes extreme, so that it is impossible to disregard the influence of this rapid torque level variation. This rapid torque level variation causes the occurrence of vibrations or noise in the motor revolution. In addition, for a brushless DC motor for a power steering apparatus, during gradual operation of a steering wheel, the rapid torque level variation not only influences the steering feel but also causes the generation of noise.
Furthermore, in the excitation current waveform shown in FIG. 3, if the positive side (forward current) drive duty ratio (for example, taken as Duty1, and referred to hereinafter as an xe2x80x9cupper drive duty ratioxe2x80x9d) differs from the negative side (counter current) drive duty ratio (for example, taken as Duty3, and referred to hereinafter as a xe2x80x9clower drive duty ratioxe2x80x9d), in the case that one current detecting circuit is provided in the motor drive circuit, since the rates of change at the rise and fall of two phase currents at the phase switching differ from each other, the currents for the other phases not being in the switched condition vary largely at the phase switching, so that the current variation produces transient torque variation.
The present invention has been developed in consideration of the above-described circumstances, and it is an object of the invention to provide a motor drive control system capable of suppressing current variation causing the occurrence of torque variation with a simple circuit arrangement and without use of 2 or more current detecting circuits. Further, another object of the present invention is to provide a motor drive control system capable of, in the case of controlling the drive of a brushless DC motor with rectangular waves, suppressing torque variation producing rapid level variation, and normal suppressing torque variation.
The present invention relates to a motor drive control system which controls the drive of a motor having a plurality of excitation phases, and the motor drive control system comprises a drive means for producing an exciting signal to be supplied to each of the excitation phases of the motor and a control means for determining a direction of the exciting signal for each of the excitation phases and for conducting an ON/OFF switching operation, with the control means being made to control a rate of change of the exciting signal to be switched at the switching operation.
Furthermore, the present invention relates to a motor drive control system which controls a rate of change of a current for commutation phases to prevent variation of the sum of currents of the commutation phases in exciting coils of a brushless DC motor, wherein a time period (energizing period), for which a drive current is supplied to an OFF-phase in the exciting coils according to a pulse width modulation, is limited to eliminate a residual current for the OFF-phase of the commutation phases in the exciting coils.
The present invention relates to a motor drive control system which controls the drive of a motor having a plurality of excitation phases without using two or more current detecting circuits for detecting exciting currents in the motor, and the motor drive control system comprises a drive means for producing an exciting signal to be supplied to each of the excitation phases of the motor and a control means for determining a direction of the exciting signal for each of the excitation phases and for conducting an ON/OFF switching operation, with the control means producing the exciting signal to maintain the sum of the exciting currents for the excitation phases in the motor constant at the switching operation.